1. Field of the Invention.
The invention relates to integrated circuits and more particularly critical timing in self timed circuits and other circuits where one event is intentionally delayed from a second event.
2. Prior Art.
Often in integrated circuits an event is initiated by the occurrence of another event and a predetermined delay. For instance, sense amplifiers in some static random-access memory (RAM) are enabled after a delay measured from activation of the word lines in the memory. In this case, it is desirable to have the sense amplifiers enabled as soon as possible after the word lines are activated in order to more quickly read data from the memory. However, the sense amplifiers cannot be activated too soon after the word lines become active since sufficient time must lapse for the bit lines to become sufficiently different in potential otherwise the sense amplifiers cannot sense the data. Numerous well-known self timed circuits are used for enabling the sense amplifier as well as other events in these memories. See, for example, U.S. Pat. Nos. 4,926,387 and 5,031,141.
Sometimes the self timed circuit, such as a sense amplifier, is not well characterized. While simulations are generally used to design the circuit such techniques do not account for all the factors determining the operating characteristics of the circuit. Thus, for example, the simulation may not accurately account for the influences from the surrounding circuitry in the final layout of the integrated circuit. Margins and guard bands are designed in to compensate for such things as process variations and inaccuracies in the simulation. After the integrated circuit is fabricated, if no failures occur in the self timed circuit, generally, there is no need to characterize the circuit. Consequently, it is not known how much of the margins or guard bands are really needed to assure a high yielding design. For instance, a simulation may show that a sense amplifier will operate when the differential between the bitlines is approximately 60 mvolts yet 160 mvolts may be used for selecting the time delay for activating the sense amplifier. This may be looked at as the price of ignorance. When these margins and guard bands are in a critical path in an integrated circuit they are particularly costly.
It is a difficult, if not impractical, task to characterize many circuits. In the case of the sense amplifier for one, there is no direct access to this circuit that would permit testing. Direct access by probing changes the characteristics and prevent an accurate evaluation.
A method and/or apparatus for easily and accurately characterizing a circuit in an integrated circuit is needed. As will be seen, the present invention provides a method and apparatus for building in a test feature which permits the characterization of the self timed circuit.